1. Field of the Invention
This invention relates to tooling for forming a hem between a pair of overlapped metal pieces, that is, for deforming an edge of one of the metal pieces to overlie the other metal piece in a continuous or nearly continuous pattern.
2. Description of the Prior Art
Hemming is a technique that is widely used in the automotive industry for joining a sheet of metal that serves as an external body component to a formed piece of metal that serves as a reinforcing element for such external body component. For example, the trunk deck lid of most front engine automotive vehicles is of two-piece construction in which the outer edge of the outer element of the trunk deck lid is folded over against the outer edge of an inner reinforcing element by a hemming process. Devices for performing hemming operations of the type described are shown in U.S. Pat. No. 4,346,579 to Takatsu and U.S. Pat. No. 4,484,467 to Kitano, et al.
The hemming process, as described, normally also involves the application of a thermosetting organic sealing compound between the overlapped edges of the inner and outer elements, to help prevent laceration injuries to persons who may grasp the trunk deck lid during opening or closing, since the exposed edge of the rolled over portion or hem of the outer metallic element can be rather sharp. Many front engine automotive vehicles also utilize a two-piece hood in which the outer hood element is reinforced by an inner structural element and in which the outer edge of the outer element is folded over the outer edge of the inner element by hemming, again, with the addition of an organic sealing compound before the hemming step to cover the exposed sharp edge of the outer metallic element.
Hemming processes as heretofore described utilize an outer element with the outer edge prefolded in the form of a flange to lie approximately perpendicularly to the main portion of the outer element, such prefolding being done most conveniently in the stamping operation that is customarily utilized in the forming of such outer element. The hemming of such flange requires that it be folded over from such prefolded condition approximately ninety degrees (90.degree.), to be against the outer edge of the inner element. after the inner element, whose main portion extends generally parallel to the main portion of the outer element, has been placed inside the flange of the outer element. The folding over or hemming of the flange of the outer element in many hemming processes of the prior art is done in multiple stages, usually in two stages, in which, in a first stage, force is applied generally perpendicularly to the original orientation of the flange to cause it to bend approximately thirty-five to fifty-five degrees (35.degree.-55.degree.) from its original orientation, and in which, in a second stage, force is applied generally parallel to the original orientation of the flange to cause the partially bent flange to bend an additional approximately fifty-five to thirty-five degrees (55.degree.-35.degree.) to complete the approximately ninety degrees (90.degree.) of folding of the flange from its prefolded condition to securely engage the outer edge of the inner element of the two-piece structure that is being hemmed. Such a two-stage hemming process is done in separate sets of tooling, tooling which is rather massive, costly, and space-consuming, and a two-stage hemming process requires a transfer operation to transfer the workpieces that are being hemmed, in unison, from the first stage tooling to the second stage tooling. Such a transfer operation involves special transfer equipment, an additional cost factor, and poses additional risks of equipment malfunction which can lead to production interruptions. Multiple stage hemming operations of the aforesaid type also require, for process considerations, a certain minimum depth of flange in the outer edge flange of the outer element that exceeds the depth of the flange that would otherwise be required based on the product requirements of the component that is being hemmed, and to the extent that the flange depth required for process considerations exceeds the flange depth required for product considerations, the finished component is more costly and more heavy than it would otherwise need to be.
The advantages of performing an entire hemming operation in a single stage are recognized in U.S. Pat. No. 3,191,414 to Kollar, et al, which describes a hemming tool that is actuated sequentially in horizontal and vertical directions by separate hydraulic cylinders acting through a linkage system, and in U.S. Pat. No. 3,276,409 to St. Denis. The structures of the Kollar, et al and St. Denis patents are structurally and hydraulically complex, however, especially since a typical automotive trunk or hood hemming station requires the use of several hemming tools arranged end-to-end around the perimeter of the parts that are joined to one another. Possibly because of the complexity of the hemming tooling of the aforesaid Kollar, et al and St. Denis patents, single stage hemming of large parts, such as automotive hoods and trunk deck lids, has not heretofore proven to be successful, and is not known to be in commercial practice, at least to any appreciable extent.